The present invention relates to refrigeration chillers, to the compressors by which they are driven and to the lubrication thereof. With still more particularity, the present invention relates to refrigeration chillers driven by screw compressors and apparatus by which to ensure the immediate availability of lubricant to the compressor at chiller start-up.
The primary components of the refrigeration circuit of a refrigeration chiller include a compressor, a condenser, an expansion device and an evaporator. High pressure refrigerant gas is delivered from the compressor to the condenser where the refrigerant gas is cooled and condensed to the liquid state. The condensed high pressure refrigerant passes from the condenser to and through the expansion device. Passage of the refrigerant through the expansion device causes a pressure drop therein and the further cooling thereof. As a result, the refrigerant delivered from the expansion device to the evaporator is cool and is at relatively low pressure.
The refrigerant delivered to the evaporator is brought into heat exchange contact with a tube bundle disposed therein through which a relatively warmer heat transfer medium, such as water, flows. That medium will have been warmed by heat exchange contact with the heat load which it is the purpose of the refrigeration chiller to cool.
Heat exchange contact between the relatively cool refrigerant and the relatively warm heat transfer medium in the evaporator causes the refrigerant to vaporize and the heat transfer medium to be cooled. The now cooled medium is returned to the heat load to further cool it while the now heated, low pressure refrigerant is drawn out of the evaporator and into the compressor in the gaseous state for recompression and delivery to the condenser in a continuous process.
Where the compressor by which a refrigeration chiller is driven is a compressor of the screw type, it is typical that a relatively large amount of compressor lubricant will mix with the refrigerant gas undergoing compression therein and will be carried out of the compressor entrained in the stream of high pressure refrigerant gas discharged therefrom. To a somewhat lesser extent this is also the case in chillers driven by compressors of other than the screw type.
An oil separator will typically be disposed downstream of a screw compressor in a refrigeration chiller for the purpose of disentraining lubricant from the high pressure refrigerant gas in which it is carried out of the compressor. The disentrained oil settles into a sump within the oil separator. The relatively high pressure that exists within the oil separator is used to drive the disentrained lubricant from the sump back to the compressor for purposes such as bearing lubrication, sealing and cooling of the refrigerant gas undergoing compression therein.
Because the disentrained oil is exposed to the relatively high discharge pressure that exists in the oil separator and because it is at relatively high temperature, it will typically absorb and contain on the order of 30% by weight of the refrigerant from which it has been disentrained. When a screw compressor-driven refrigeration chiller is shut down under certain operating circumstances, particularly when operating at or near full load and such as during a power interruption or an emergency stop, the resulting precipitous pressure drop in the high pressure side of the chiller system causes the relatively violent outgassing of the absorbed refrigerant from the oil on that side of the system as well as the gas-driven reverse direction high speed rotation of the no longer motor-driven screw rotors. These effects result from the system's attempt, once it shuts down, to equalize pressures within itself across the compressor and expansion devices which generally define the boundaries of the high and low pressure sides of the refrigeration circuit within a chiller when it is in operation. Under such circumstances, the main oil line connecting the compressor and the sump in the oil separator can be blown dry.
Under such shutdown circumstances, provided that the conditions causing them are transient, the chiller system will attempt to restart relatively quickly after shutting down. If the oil feed line to the compressor has been blown dry, such re-starts can be unsuccessful due to the lack of a sensed supply of oil in the compressor supply line or can, if successful, potentially have the long term effect of damaging the compressor for intermittent lack of lubricant at start-up.
The need exists, in order to assure the long-term reliability of the compressor and to reduce or eliminate repeated unsuccessful attempted chiller re-starts and the service calls that can result therefrom under certain circumstances, for apparatus and/or a method by which to assure lubricant flow to a screw compressor in a refrigeration chiller shortly after chiller start-up even if the nature of the preceding chiller shutdown was such as to cause the oil supply line to the compressor to be blown dry.